The social control of sex reversal in group-living species of hermaphroditic fish provides an excellent system for the causal analysis of the influence of social factors on the physiological status of the individual and of various features of social interaction and group structure. Anthias squamipinnis is a small, sexually dimorphic coral reef fish which occupies stable, sedentary social groups. All individuals mature as females. Males are produced solely by the sex reversal of adult females, a process initiated by the removal of a male from a social group. I propose to perform a three-month field study to test five hypotheses concerning the causal mechanism by which alterations in male-female interactions induced a female to change sex. Methods involve removing selected individuals from experimental and control groups which have been matched for composition and structure, and subsequently observing the groups for the appearance of sex reversal. The results are expected to show that females do not have a continual tendency to change sex which is suppressed by the presence of a male, that sex reversal is initiated by a critical quantitative increase in a particular behavioral measure, and that sex reversing individuals are influenced by multiple intra-group social events which occur synchronously in far-removed portions of the group. Because social groups of these fish are highly structured spatially and show patterns of behavioral interaction similiar to those of primates, the processes revealed by this study should be highly relevant to an understanding of general processes by which social interactions affect the behavioral and physiological status of the individual living in a group. By helping to reveal principles by which groups are structured, this study will increase our understanding of the processes underlying human mental health.